The process and apparatus of present invention concern the thermal cycles of molding expandable polystyrene (hereinafter "EPS") into useful articles made of what is commonly called "styrofoam".
Known methods for molding EPS begin with the step of filling a mold with EPS beads. The beads are small hollow polystyrene spheres filled with a gas expansion agent. The molds are usually made of cast aluminum and consist of two halves. Each half is mounted onto a platen to form a "mold assembly" and create a steam cavity behind each side of the mold. Generally, one or both of the platens is moveable to allow separation of the two sides of the mold and thereby facilitate removal of the item being produced.
Once the mold is filled with beads, the next step in EPS molding is a heat cycle to heat the mold and beads to expand and fuse the beads to form the product. The stem cavity behind one half of the mold is filled with steam This steam is drawn through core vents into the bead-filled mold cavity by applying a vacuum behind the other half of the mold. The direction of flow of the steam can be reversed by switching the location of the source of steam and the vacuum being applied.
U.S. Pat. No. 4,272,469, to Smith, describes this type of heating cycle wherein the flow of steam lasts "a period of time" before reversing its direction. (Column 2, line 65--column 3, line 4 and column 8, lines 12-13). The Smith heating cycle takes two minutes or longer, as seen in Examples 1 and 2. (Column 8, lines 23-62). U.S. Pat. No. 4,557,881, to Rabotski also describes this type of heating cycle wherein the steam flows for ten seconds in each direction and is then supplied from both sides of the mold for twenty seconds. (Column 8, lines 44-54). Both Smith and Rabotski rely on a timed cycle to determine when the beads have been sufficiently heated, rather than a measurement of the temperature of the beads.
When the heat cycle is complete, a cooling cycle is initiated. Cooling the mold and product therein brings internal bead pressure near zero thereby preventing expansion and facilitating ejectment. In Rabotski, many gallons of cold water are sprayed onto the mold halves to cool the mold, and hence the product therein, before the product is ejected from the mold. (column 9, lines 9-30). Heat sensors monitor the temperature of the cooling water and regulate the length of the cooling cycle. This method of cooling will generally require a re-heating of the mold before subsequent heating cycles are initiated. The evaporation of water which may be present within the product due to condensation of the steam used for heating is another recognized method for cooling the product. See, for example, U.S. Pat. No. 3,015,851 to Wiles at column 6, lines 22-25, U.S. Pat. No. 3,312,760 to Berner at column 2, lines 5914 64 and U.S. Pat. No. 4,272,469 to smith at column 8, lines 38-48.
Other relevant references include: U.S. Pat. No. 3,042,967 to Edberg; U.S. Pat. No. 3,264,381 to Stevens; U.S. Pat. No. 3,274,643 to Oxel; and U.S. Pat. No. 4,439,122 to Besse et al.
In present practice of the prior art processes a total cycle time for producing EPS articles is no less than 90 to 120 seconds and is often longer. Further, present prevailing practice requires tremendous energy expenses to heat the mold and its contents, subsequently, cool the mold and then reheat the mold to begin a new cycle. The vast majority of the energy requirement is to heat the mold which has a much denser mass than the EPS product inside, thus, requiring additional heat energy for its heating and reheating for each cycle.
The method and apparatus of the present invention represent improvements over these prior art processes by reducing energy requirements and greatly reducing the length of both the heating and cooling cycle. The present invention represents an important and exciting new process because it avoids the energy wasteful heating and reheating of the mold parts by instead concentrating the heat requirements directly on the product materials. The product materials are traditionally less dense and therefore less heat energy demanding than the mold materials and by heating the product materials directly heat requirements are reduced without any loss of process performance. The present invention, which utilizes a direct method for heating and cooling the product, permits a revolutionary reduction in process cycle time from the traditional minimum of 90 to 120 seconds to a cycle time of 24 seconds or less. This reduced time requirement contributes to energy efficiency and increases time output plant product. Even further energy efficiency is achieved because the requisite amount of steam required for molding can be precisely determined in accordance with preferred methods of the invention thus reducing unnecessary waste.